This invention relates to scintillators for use in neutron detection and neutron detectors that use them to detect neutrons. The invention is characterized by the use of a scintillator that has small fluorescent yield but with a very short decay time so that neutrons of high incident rate can be detected. The invention is also characterized by its ability to acquire a two-dimensional neutron image at very high speed.
In particular, the invention enables two-dimensional neutron imaging at high counting rate and thus makes great contribution to the advancement of studies in materials physics and structural biology based on neutron scattering and the like that rely on the time-of-flight (TOF) method using pulsed neutrons generated by a high-intensity positron accelerator. The invention can also be employed in similar studies using a nuclear reactor as a neutron source. In addition, the invention will make great contribution to the advancement of studies in materials physics and structural biology based on X-ray scattering and the like that employ high-intensity synchrotron radiation. Because of having high sensitivity to neutrons, the invention can also be applied to instrumentation for monitoring neutrons from nuclear reactors or as neutron dose monitors.
Conventionally, Li glass scintillators or LiI(Eu) scintillators have been used as neutron detectors or neutron imaging detectors. These scintillators for neutron detection have reasonably high fluorescent yield but they also have high sensitivity to gamma rays. They also have long decay times (60 ns for the Li glass scintillator and 1.4 μs for LiI(Eu)) and have had difficulty achieving measurement at high counting rate (≧10 Mcps) (Knoll, “Radiation Detection and Measurement”, Japanese translation made by Kimura and Sakai and entitled “Hoshasen Keisoku Handbook”, 2nd ed.)
The Li2B4O7 single crystal is referred to in a review article written by C. W. E. van Eijk about scintillators for use in neutron detection [Nucl. Instr. and Meth. A460(2001)1–14]. I. K. Kamenskikh et al. (SCINT 97/1997, P. 65) performed fluorescence measurement with uv radiation and confirmed that a wavelength spectrum for fluorescence peaked at 340 nm. However, no measurement has yet been made of the neutron-dependent fluorescence characteristics or the characteristics for neutron detection by the combination of the scintillator with a photomultiplier tube and the neutron-dependent decay time which is the most important in neutron detection has not been measured, either.
With the recent use of high-intensity pulsed neutron sources that are created in high-intensity positron accelerators, more intense pulsed neutrons are produced and the produced neutrons have a wider energy spectrum; this makes it essential to develop a detector that will not be saturated even at high counting rate but which can detect neutrons and produce a two-dimensional neutron image in a simple way. Also indispensable is neutron detection with reduced gamma-ray background and it has been desired to develop a scintillator for neutron detection that is composed of as light elements as possible. As a further problem, if the fluorescent yield is unduly high, neutrons coming in at high concentration within a short time will cause the scintillator to emit such a large quantity of fluorescence that the photomultiplier tube is saturated temporarily. It has therefore been desired to develop a scintillator that has a very short decay time (≦10 ns) with reasonable fluorescent yield.